The apoproteins of serum alpha-lipoprotein and beta-lipoprotein have been found to be major intrinsic protein of the human erythrocyte membrane. The presence of these apoproteins in myelin and the plasma membranes of fat cells, cultural fibroblasts, and muscle cells will be investigated. Incorporation of I125-labelled alpha- and beta- lipoproteins into the plasma membranes of erythrocytes, fat cells, and cultured fibroblasts will be studied. The incorporation of C14-D-glucosyl isothiocyanate, a covalent affinity label for the glucose transport proteins of the human erythrocyte, into the plasma membrane of fat cells and muscle cells will be assessed. Similar studies will be carried out with maltasyl isothiocyanate.